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GPS: Where in the World Are You?

In this interactive activity produced for Teachers' Domain, learn how the Global Positioning System (GPS) uses trilateration to identify the location of a receiver. Investigate how each GPS satellite broadcasts a radio signal that carries information (the time the signal was transmitted, the position of the satellite, and a pseudo-random-noise code) and how the signals from three different satellites are used to determine the receiver's location. In addition, explore how the signal from a fourth satellite is used to achieve greater precision.

The Global Positioning System (GPS) provides accurate navigation and positioning information worldwide. The orbital patterns of this collection of at least 24 satellites guarantee that at any given moment, from any place on Earth, there is an unobstructed view of 4 or more satellites. Each satellite orbits in a known path and broadcasts a unique signal. The GPS receiver uses this information from at least 3 satellites to calculate its own location, using the mathematical method of trilateration.

In everyday language, when people refer to "GPS," they are most likely referring to a GPS receiver. The various GPS devices that we have come to know, such as car navigation systems and mobile phones, are actually GPS receivers that function as only one part of a system that includes the receiver and a larger constellation of satellites. A GPS receiver analyzes the radio signals that are broadcast from the satellites; because the signals travel at the speed of light, the receiver can calculate the distance to the satellite by measuring the time it takes for the signal to arrive. The receiver uses the information from a minimum of three satellites to determine its location. The more satellites used, the more precise the location.

The navigational, tracking, mapping, and surveying applications for GPS technology have extended far beyond their initial military uses. Now GPS can be used by anyone for navigation on land, on sea, and in the air. Drivers in cars are never lost when they have a GPS device guiding them through the streets, recreational boaters know where they are even when they can't see land, and pilots have detailed information not only on the aircraft's latitude and longitude but on altitude and velocity as well. The information provided by GPS can help make air travel more efficient and economical by helping to keep pilots on the most direct route possible between two locations, saving time and fuel.

There are a seemingly endless number of ways in which GPS can be used. For example, runners wearing a GPS watch can keep a training record of their route, pace, and distance traveled. GPS allows businesses to track packages or monitor their fleet of emergency response vehicles. In addition, GPS technology is valuable for gathering scientific data, such as tracking animal migration patterns or surveying glacier movement. It can even be used in tsunami warning systems. Traditional tsunami warning systems often generate false alarms, but GPS devices stationed at coastal areas near the epicenter of an earthquake give details about the horizontal and vertical displacement at a specific location on the seafloor, improving the speed and accuracy of analyses and making warnings more reliable.